In our structural studies of viral infections of nerve cells, we have demonstrated: (1) In vivo, the restricted tropism for neurons of a mutant of the wild JHM strain of a mouse coronavirus. (2) In vitro, a hepatotropic and a neurototropic mutant of JHM strain have restricted tropism for neuron and the mutant shows abnormal assembly in CNS cells. (3) A wild measles strain can produce a selective infection of mouse neurons in the process of differentiation and the host cell is responsible for the establishment of virus persistence. Infected neurons appear to be unable to redistribute measles antibody complexes in their surface. In our studies on formation of myelin, we have demonstrated that : (1) P2, a peripheral nervous system protein, is also localized in rabbit oligodendroglial and CNS myelin. (2) The major peripheral nerve glycoprotein, Po, is localized on the cytoplasmic side of the Schwann cell plasma membrane, the outer mesaxons and the major dense line of compact myelin. The Golgi system and some cytoplasmic vesicles of Schwann cells also contain Po. (3) The myeline associated glycoprotein, MAG, is localized in the Schmidt-Lanterman incisure, the paranodal area and the outer mesaxons of actively myelinating Schwann cells. (4) In vitro, fibronectin promotes rat Schwann cell growth and motility.